Testing topography and fault geometry effects on earthquake ruptures and ground motions along double compressional bends

Nicholas Madera, & Julian C. Lozos

Published September 10, 2023, SCEC Contribution #12954, 2023 SCEC Annual Meeting Poster #120 (PDF)

Fault geometry is known for affecting the endpoints of earthquake ruptures. Recent studies suggest that ground surface topography also affects earthquake ruptures and ground motion, but they do not offer much physical explanation for this. This project is a dynamic rupture modeling geometrical parameter study that continues to explore the effects of fault geometry and topography on earthquake ruptures and ground motion on double compressional bends (DCBs). DCBs are nonplanar strike-slip faults that consist of two parallel fault segments connected by a linking segment at some angle. Transpression occurs along the linking segment statically due to the orientation within a regional stress field and/or dynamically due to the sense of slip on the fault. This transpression forms mountains on both sides of the linking segment. Notable examples of DCBs on the San Andreas Fault in California are the Big Bend in Southern California and the Santa Cruz Mountains in Northern California. 1857 Fort Tejon and 1906 San Francisco earthquakes have ruptured into their local DCBs, whereas the 1989 Loma Prieta earthquake remained confined to the linking segment of the Santa Cruz Mountains. Our primary variables are fault bend angles, and mountain height and width. We create the meshes in Coreform Cubit and run our models in a 3D finite element dynamic rupture simulation software called FaultMod. The results from 142 models show that bend angles have a larger role in controlling earthquake ruptures than topography does. For fault bend angles greater than or equal to 23 degrees, earthquakes stop within the linking segment, while faults with bend angles less than or equal to 22 degrees rupture entirely, regardless of topography. Interestingly, topography does allow the fault to slip further compared to the flat models. For faults with bend angles greater than or equal to 24 degrees, topography widths less than or equal to 1000 m, and topography heights greater than or equal to 800 m, a tail of surface slip is present past where the rupture stops at depth (main slip). This tail of surface slip is disconnected with the main slip at 30 degrees bend angles but connected with the main slip at 25 degrees and 24 degrees. For models with the same topography height and width, faults with lower bend angles had longer rupture lengths than faults with larger bend angles. We are still working on interpreting the physical processes that control these results.

Key Words
dynamic rupture modeling, earthquake models, earthquake simulations, strike-slip faults, rupture dynamics, fault geometry, ground motion modeling, parameter study

Citation
Madera, N., & Lozos, J. C. (2023, 09). Testing topography and fault geometry effects on earthquake ruptures and ground motions along double compressional bends. Poster Presentation at 2023 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)